An exhaust system conducts hot exhaust gases generated by an engine through various exhaust components to reduce emissions and control noise. The exhaust system includes an injection system that injects a diesel exhaust fluid (DEF) or a reducing agent, such as a solution of urea and water for example, upstream of a selective catalytic reduction (SCR) catalyst. A mixer is positioned upstream of the SCR catalyst and mixes engine exhaust gases and products of urea transformation.
The mixer includes an upstream baffle with an inlet opening and a downstream baffle with an outlet opening, with the upstream and downstream baffles being surrounded by an outer peripheral wall. A doser is mounted at an opening formed within the outer peripheral wall to spray the DEF into the exhaust stream. The spray tends to have a larger droplet size in order to provide a high spray penetration such that the droplets are spread across the entire mixer cross-section. However, smaller droplet sizes are preferred as droplets with smaller diameters evaporate more quickly than larger diameter droplets.
Due to packaging constraints it is important to provide a compact configuration for the mixer by minimizing the length of the mixer. Typically, mixers having a short length also have a large exhaust flow cross-sectional area. This is disadvantageous when using smaller droplet sizes as the droplets tend not to penetrate sufficiently into the exhaust flow. Thus, having the desired shorter mixing length and the desired smaller spray droplet size are at odds with each other. Not only must the droplets have a high spray penetration, the droplets must be distributed evenly.
In one known compact mixer configuration set forth in U.S. Pat. No. 8,661,792, the inlet and outlet openings are orientated relative to each other such that the flow path provides 300 degrees of rotation from the inlet opening to the outlet opening. While this degree of rotation facilitates penetration and distribution, it is desirable to further improve mixing performance without increasing the mixer length.